1. Field of the Invention
The present invention relates, in general, to spacer grids used in nuclear reactor fuel assemblies to place and support a plurality of longitudinal fuel rods in the reactor fuel assemblies and, more particularly, to a spacer grid for pressurized water reactor fuel assemblies, which has a grid spring at a central portion of each unit strip of perimeter strips so that the grid springs of the perimeter strips are in equiangular surface contact with the fuel rods in a pressurized water reactor fuel assembly, thus reducing fretting corrosion of the fuel rods, and which has both guide vanes and guide taps on upper edges of some unit strips of the perimeter strips that are required to guide cross flows of the coolant in the reactor fuel assembly, thus maintaining desired intensity of cross flows of the coolant in the reactor fuel assembly and improving the thermal redundancy of the spacer grid.
2. Description of the Related Art
Spacer grids are elements of a nuclear reactor fuel assembly, and each have a plurality of grid springs and dimples in their support cells so as to place and support a plurality of longitudinal fuel rods within the cells of the reactor fuel assembly. As shown in FIG. 1, a typical nuclear reactor fuel assembly 101 is fabricated with a plurality of guide tubes 113 and a plurality of spacer grids 110. The plurality of guide tubes 113 are vertically arranged between a top support pallet 111 and a bottom support pallet of the reactor fuel assembly 101, while the plurality of spacer grids 110 to place and support a plurality of longitudinal fuel rods 125 in the fuel assembly 101 are horizontally arranged along the guide tubes 113 at regular intervals in a longitudinal direction of the fuel assembly 101. The spacer grids 110 are typically mounted to the guide tubes 113 through a welding process.
Each of the spacer grids 110 includes a plurality of inner strips 115 and four perimeter strips 116. The plurality of inner strips 115 are arranged while intersecting each other at right angles to form an egg-crate pattern, prior to being encircled with the four perimeter strips 116. The inner and perimeter strips 115 and 116 are made of a zircaloy alloy, and respectively have a plurality of grid springs 150 and 120 to place and support the plurality of fuel rods 125 in the fuel assembly, as shown in FIGS. 2A and 2B. However, the inner and perimeter strips 115 and 116 do not have any guide vane specifically designed to guide cross flows of the coolant in the fuel assembly or any guide tap specifically designed to prevent interference of the strips 115 and 116 with the fuel rods 125.
When the grid springs 150 and 120 of the inner and perimeter strips 115 and 116, and a plurality of dimples 170 of the inner strips 115 have deficient spring forces, the spacer grids 110 may fail to stably place or support the fuel rods 125 at desired positions in the fuel assembly, thus reducing the soundness of the fuel assembly. On the contrary, when the grid springs 120 and 150 and the dimples 170 are too strong in their spring forces, excessive friction may be generated between the fuel rods 125 and the spacer grids 110 during an insertion of the fuel rods 125 into the cells of the spacer grids 110. Such excessive friction may cause damage, such as scratches, to the external surfaces of the fuel rods 125, and fail to appropriately support the fuel rods 125 in the case of lengthwise growth of the fuel rods 125 caused by neutron radiation during an operation of a nuclear reactor. In such a case, the fuel rods 125 may be undesirably bent. When the fuel rods 125 are bent as described above, the fuel rods 125 in the reactor fuel assembly become closer to each other to be sometimes brought into undesired contact with each other, so that coolant channels defined between the fuel rods 125 in the fuel assembly may become narrower or even closed. In the above state, heat cannot be efficiently transferred from the fuel rods 125 to the coolant, thus parts of the fuel rods 125 may be overheated, and sometimes cause a DNB (Departure caused by Nucleate Boiling) to reduce the output power of nuclear fuel.
The recent trend of development in the reactor fuel assemblies aims at the provision of highly combustible and defect-free nuclear fuel. Particularly, to provide the highly combustible nuclear fuel, the heat transfer efficiency between the fuel rods and the coolant in the reactor fuel assembly must be enhanced. This means that the heat transfer from the fuel rods to the coolant in the reactor fuel assemblies must be improved to enhance the thermal efficiency of the reactor fuel assemblies. The improvement in the heat transfer from the fuel rods to the coolant may be accomplished by designing the reactor fuel assembly to allow the coolant to optimally flow around the fuel rods in the reactor fuel assembly. In an effort to accomplish the optimal flows of the coolant within the reactor fuel assembly, several types of spacer grids having new structures have been proposed. For example, to provide the optimal flows of the coolant in the reactor fuel assembly, a plurality of mixing blades 227 may be attached along an upper edge of each inner strip 215 of a spacer grid at intersections of the inner strips 215, as shown in FIGS. 3A and 3B, so as to mix the coolant and generate cross flows of the coolant between neighboring coolant channels in the fuel assembly. To mix the coolant and generate the cross flows of the coolant between the neighboring coolant channels, the mixing blades 227 are curved in opposite directions, and the size and angles of the mixing blades 227 are optimally determined. The mixing blades 227 provided on the spacer grid thus maintain a desired coolant flow pattern relative to neighboring spacer grids.
However, the above-mentioned conventional technique for accomplishing the optimal flows of the coolant in the fuel assemblies by use of the mixing blades 227 and thereby enhancing the thermal efficiency of the fuel assemblies is based on the formation of more active turbulent flows of the coolant around the fuel rods of the fuel assemblies. Therefore, the conventional technique undesirably induces vibration of the fuel rods in the fuel assemblies due to the active turbulent flows of the coolant. Such vibration of the fuel rods in the reactor fuel assemblies is a so-called “flow-induced vibration”. Due to the flow-induced vibration of the fuel rods in the reactor fuel assemblies, the fuel rods slide or move relative to the grid springs and dimples at contact surfaces thereof. The fuel rods are thus abraded on the contact surfaces due to friction. The flow-induced vibration of the fuel rods thus finally causes “fretting corrosion of the fuel rods”. The conventional technique for enhancing the thermal efficiency of the reactor fuel assemblies and providing highly combustible nuclear fuel may undesirably cause damage to the fuel rods.
While designing the spacer grids for reactor fuel assemblies, it is necessary to accomplish the following three requirements: 1) the spacer grids must stably support the fuel rods until the expected life span of the fuel rods expires; 2) the spacer grids must be free from causing fretting corrosion of the fuel rods; and 3) the spacer grids must have outermost cells having higher structural durability.
In a detailed description, first, to allow the spacer grids to stably support the fuel rods within a reactor fuel assembly until the expected life span of the fuel rods expires, and accomplish the soundness of the fuel assembly, the spacer grids must be designed such that the spacer grids effectively support the fuel rods with sufficient spring force of the grid springs and dimples thereof. In addition, the elastic range of the grid springs and dimples must be enlarged, thereby maintaining a desired spring force regardless of variable fuel rod support conditions inside the reactor fuel assembly until the expected life span of the fuel rods expires. However, the grid springs and dimples of a conventional spacer grid for the reactor fuel assemblies gradually lose the original spring forces thereof, due to a neutron radiation during an operation of a nuclear reactor. Therefore, the grid springs and dimples may fail to desirably support the fuel rods, thus there may be formed gaps between the fuel rods and both the grid springs and the dimples. Due to the gaps, the spacer grids do not stably support the fuel rods, but undesirably allow the fuel rods to be excessively loaded and move in every direction by the flows of the coolant. The spacer grids thus reduce the soundness of the reactor fuel assemblies.
Second, the protection of the fuel rods from the fretting corrosion in the reactor fuel assembly may be accomplished by removing the causes of the fretting corrosion. The causes of the fretting corrosion of the fuel rods in the reactor fuel assembly include gaps formed between the grid springs, the dimples and the fuel rods. The gaps may be formed by a reduction in the spring force of the grid springs and dimples due to the neutron radiation during the operation of the nuclear reactor, a difference in the thermal expansion between the fuel rods and the spacer grids, and a reduction in the diameter of the fuel rods caused by an elongation of the fuel rods. When the gaps are formed between the grid springs, the dimples and the fuel rods, the fuel rods are repeatedly brought into contact with and spaced away from the grid springs and the dimples, due to axial and transversal flows of the coolant, thus causing the fretting corrosion of the fuel rods in the reactor fuel assembly.
Third, the outermost cells which are defined in each spacer grid along the perimeter strips, must endure hydraulic loads caused by the cross flows of the coolant generated in the coolant channels defined between the outermost cells and neighboring fuel rods and/or generated in the spaces between the outermost cells and a variety of internal structures of the nuclear reactor. Therefore, the outermost cells of the spacer grid must be designed to endure the maximum load higher than the maximum load to be endured by the inner cells that are defined in the spacer grid by the intersecting inner strips. Furthermore, the outermost cells of the spacer grid must have a behavior capable of sufficiently enduring an excessive load unexpectedly applied thereto due to carelessness while handling the fuel rods, so that the outermost cells must be designed to have the higher structural durability.